For more than thirty years, vascular diseases have been treated using open surgical procedures. In 1999 alone, 753,000 open-heart procedures, including coronary artery bypass grafting (CABG), valve replacements, and heart transplants, were performed. During a typical CABG procedure, a sternotomy is performed to gain access to the pericardial sac, the patient is put on cardiopulmonary bypass (CPB), and the heart is stopped using a cardioplegia solution.
Generally, previously-known CPB is accomplished by constructing an extracorporeal blood handling system including, inter alia, a venous line, a venous reservoir, a centrifugal or roller pump that perfuses blood through the extracorporeal circuit and the patient, an oxygenator for oxygenating the blood, an arterial line for returning oxygenated blood to the patient, and an arterial filter located in the arterial line. The pump in previously known methods of CPB is placed after the venous reservoir and the venous flow into the reservoir is driven by negative pressure in this line from a siphon and not the pump. In order to minimize the diameter and cannula size required for the venous line, vacuum is often applied to the reservoir, as described, for example, in U.S. Pat. No. 6,017,493 to Cambron. The use of a venous reservoir provides compliance in the blood treatment system so the venous flow may be controlled independently of the arterial or return flow to the patient.
Previously-known methods of CPB are susceptible to several error or trigger conditions. For instance, one trigger condition is the inadvertent introduction of air into the extracorporeal circuit. This may occur in a number of ways, including inadvertent opening of a vent line, improper priming of the circuit, or by turning the heart during surgery. In addition, differences between blood inflow to a venous reservoir and outflow from the venous reservoir due to the pump head can lead to depletion of the reservoir and the entrainment of large amounts of air. If returned to the patient, air can cause significant patient injury such as brain damage, cardiac dysfunction, and myocardial damage. Further, an air-blood mixture may cause turbulence and high shear stresses within the circuit, resulting in hemolysis and humoral and/or cellular activation.
Previously known CPB systems, such as the S3 System sold by Stockert GmbH, Munich, Germany, the HL 20 Heart Lung Machine sold by Jostra Corp., The Woodlands, Tex., USA and the Sarns Modular Perfusion System 8000, sold by Terumo Cardiovascular Systems, Ann Arbor, Mich., USA, each include a level detector in the venous reservoir that slows and then stops delivery of blood to a patient if the volume of blood in the venous reservoir falls below a minimum volume. Each of these systems also includes a bubble detector that abruptly stops the pump if a predetermined number of bubbles larger than a predetermined size are detected.
The system shutdown strategy used in previously known CPB systems is designed to prevent de-priming of the venous reservoir and other components of the CPB circuit until the perfusionist can correct the problem. Due to the extended periods of time required to prime previously-known CPB systems, such a strategy is critical to avoid de-priming. Unfortunately, this strategy leads to no forward flow to the patient, with potentially serious consequences if flow is not restored promptly.
Another previously-known method for handling air entrained in the blood is described in U.S. Pat. No. 5,188,604 to Orth. The system described in that patent includes an air sensor disposed in the arterial line, a controller, and a series of solenoid-controlled valves, and a shunt circuit. If air is detected in blood passing to the arterial line, the controller actuates the solenoid-controlled valves to stop flow in the arterial line and simultaneously opens the shunt circuit to redirect the air-laden blood back into the blood treatment system. Like the previously-described CPB systems described above, the system described in the Orth patent results in no forward flow to the patient until the error condition is corrected.
Another trigger condition is low venous pressure, which may be caused by occlusions within the circuit. Low venous pressure is a known risk factor for air entrainment and may result in depletion of the venous reservoir as previously discussed, thus requiring blood delivery to the patient to be suspended while the condition is corrected or the CPB system is re-primed.
In addition, substantial occlusion of the venous line in previously-known CPB systems may provide minimal to no reaction time for the perfusionist to correct trigger conditions. For instance, should the venous return flow stop due to a trigger condition, such as detection of a large bubble, the perfusionist has only a few seconds to stop the heart-lung machine before the bubble is pumped into the patient.
Yet another problem with previously-known extracorporeal blood handling systems is the substantial suction force required for proper air evacuation due to an open air source. An open air source enables the pump to pull in large amounts of air, overwhelming the ability of an air evacuation line, if present, to remove the air.
In view of the aforementioned limitations, it would be desirable to provide an extracorporeal blood handling system that monitors and automatically modulates blood flow in response to trigger conditions thereby increasing the time available to the perfusionist to correct trigger conditions.
It also would be desirable to provide an extracorporeal blood handling system that monitors and automatically modulates system operation in response to the detection of gas in the system, to enhance the ability of an air evacuation line to remove the air and avoid de-priming the pump.
It further would be desirable to provide an extracorporeal blood handling system that automatically modulates pump speed in response to the detection of a massive air bolus in the extracorporeal blood circuit.
It still further would be desirable to provide an extracorporeal blood handling system that automatically modulates system operation in response to the detection of discrete trigger conditions, monitors such conditions, and resumes normal operation when the triggering conditions resolve.
It even further would be desirable to provide an extracorporeal blood handling system that automatically modulates pump speed in response to the detection of low venous pressures in the extracorporeal blood circuit.